Degree-Similar Graphs

TL;DR

This paper introduces the concept of degree similarity between graphs, explores its properties, provides constructions, and characterizes when certain matrix differences are similar over rational functions.

Contribution

It defines degree similarity for graphs, shows its relation to matrix similarity, and characterizes matrix similarity over rational functions using Smith normal forms.

Findings

Degree similarity implies similarity of various graph matrices.

The converse of degree similarity does not always hold.

Matrix similarity over rational functions is characterized by Smith normal forms.

Abstract

The degree matrix of a graph is the diagonal matrix with diagonal entries equal to the degrees of the vertices of $X$. If $X_1$ and $X_2$ are graphs with respective adjacency matrices $A_1$ and $A_2$ and degree matrices $D_1$ and $D_2$, we say that $X_1$ and $X_2$ are degree similar if there is an invertible real matrix $M$ such that $M^{-1}A_1M=A_2$ and $M^{-1}D_1M=D_2$. If graphs $X_1$ and $X_2$ are degree similar, then their adjacency matrices, Laplacian matrices, unsigned Laplacian matrices and normalized Laplacian matrices are similar. We first show that the converse is not true. Then, we provide a number of constructions of degree-similar graphs. Finally, we show that the matrices $A_1-\mu D_1$ and $A_2-\mu D_2$ are similar over the field of rational functions $\mathbb{Q}(\mu)$ if and only if the Smith normal forms of the matrices $tI-(A_1-\mu D_1)$ and $tI-(A_2-\mu D_2)$ are equal.